In electric railway systems it is usual practice to interpose insulators at predetermined intervals between sections of the contact wire. The sections are ordinarily supplied with electric power by a feeder system extending along the track and the section insulators isolate the sections from each other and make it possible to de-energize sections as desired for maintenance or repair work.
Section insulators are also required in alternating current systems where the contact wires are supplied from the conductors of a three-phase system extending along the track, one phase being connected to one section of the contact wire, another phase to a succeeding section of the contact wire, and the third phase to still another such of the contact wire. This is done to balance the load on the three phases of the power supply and it is essential that each section of contact wire be insulated from adjacent sections that are energized by different phases of the power supply system. The section insulators for accomplishing this purpose are called "phase breaks" in the industry.
In both ordinary section insulators and phase breaks, it is essential not only that adjacent sections of contact wire be electrically insulated from each other, but also that mechanical connections between the sections be provided to maintain the proper tension in the adjacent sections of contact wire. Also, the contact wires in adjacent sections must be supported at uniform levels and the arrangement must be such that the current collectors on the trains, such as the contact bars of the pantographs, will ride smoothly and without instability from one contact wire across the section insulator to the next contact wire without undue sparking taking place and without the imposition of undue shocks or excessive mechanical forces on the pantograph, the contact wires, or the intermediate portions of the section insulators.
It has been proposed, as in British Pat. No. 18,255 of 1906, to employ forked members connected to the opposed ends of adjacent sections of the contact wires, the members having arms which are disposed on either side of an intermediate piece having a surface of insulating material that is engaged by the collector element during its passage from one forked member to the other. This system provides air gaps between the arms of the forked terminal members and the intermediate piece. The disclosed construction, however, embodies heavy rigid parts that would be expensive to construct and install, and severe shocks would be imposed not only on the pantograph and associated elements, but also on the section insulator itself, if used in high speed service. It has also been proposed to employ a forked glider member attached to one section of the contact wire and leading to a pair of spaced runners separated by transverse air gaps from the arms of the glider and carrying the pantograph or other contactor to the contact wire of the next section, vertical members connected by strain insulators and supported from an overhead structure being employed to maintain the tension in the contact wire sections.